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Cintrón had a tough childhood. He witnessed as his mother lie in bed for months before she died of cancer. Unable to care for Cintrón and his siblings, Cintrón's father sent him to the United States, with Cintrón's uncle, Benjamin Serrano, a former Middleweight contender who had fought Frank Fletcher among others. Cintrón's father, however, kept regular contact with his kids. But when Cintrón was 13, another tragic blow shook him: His father died of a heart attack, leaving him and his siblings orphaned on both sides. He is married to María Cintrón, the couple have three children.[2] Two daughters, Denali and Savannah and a son, Santiago.[2]

In early 2008, Cintrón was trained by Emanuel Steward, who focused his training in boxing and counterattacks.[3] He decided to finish this partnership, based on the fact that Steward's time was limited due to several other compromises. Despite this, both conserved a close friendship.[3] During this timeframe, Cintrón abandoned Main Events, signing a promotional contract with Lou DiBella. His next trainer was Ronnie Shields, who emphasized on a faster training pace.[3] Shields preferred a more aggressive style, reminiscent of the one presented during the early stage of Cintrón's early career. Brian Caldwell was employed as conditioning coach, in the process modifying his weight routines.[3]

Cintrón found wrestling and boxing to be an outlet from his personal troubles, so he started spending more and more of his time practicing those sports. While attending William Tennent High School in Warminster, Pennsylvania, he became an accomplished high-school wrestler on the same team as actor Mike Vogel. After competing at William Tennent, Cintron wrestled at Thaddeus Stevens College of Technology a Junior College in Lancaster, Pennsylvania.

In April 2007, Floyd Mayweather, Jr. publicly claimed that any boxer could make the transition into mixed martial arts and win. In response, Ultimate Fighting Championship's president, Dana White, issued him a challenge to fight the promotion's lightweight champion, Sean Sherk. Mayweather later said that he did not wish to compete in the discipline. However, Cintrón stated that he was willing to fight Sherk in his place.[4] "I want the fight," said Cintrón, who was 27-1 with 25 KOs. "I can wrestle. I can box. I can beat those UFC fighters at their own game. Tell Mr. White to make me an offer and I'll take on his guy...."[4]

Cintrón did not start boxing in the amateurs until he was 19. He compiled a record of 24 wins and 5 losses as an amateur.

On the night of October 7, 2000, Cintrón knocked out Jesse Williams in two rounds in Lancaster to begin his professional career.[5] Cintrón thus began an undefeated streak that would last for more than three-and-a-half years.

One of his toughest tests during that streak came against Omar Davila on February 16, 2002.[6] Cintrón took the fight on one week's notice and traveled to his opponent's hometown of San Antonio. Despite being cut on the side of one of his eyes in the first round, Cintrón overcame adversity, coming back to defeat Davila by knockout in round two.

Cintrón was featured on NBC on May 18, 2003, against Puerto Rican veteran Luis Rosario, and he won by knockout 59 seconds into round one.

Still undefeated and now considered a rising star in the division, Cintrón was scheduled for his first world title bout against WBO welterweight champion Antonio Margarito on April 23, 2005, as part of the undercard of a Shane Mosley win by unanimous decision against the Guatemalan-American boxer David Estrada. Cintrón was dropped by Margarito four times en route to a fifth-round knockout loss.[8]

In early 2006, Cintrón rebounded with a tenth-round technical knockout of Estrada,[9] thus reestablishing himself as a contender in the welterweight division.

Cintrón then won his first title belt in a match against Mark Suarez for the IBF crown that had been vacated on June 20, 2006 by Floyd Mayweather, Jr.[10]

On July 14, 2007, Cintrón was dominant in his first title defense, knocking down Argentine Walter Matthysse three times on his way to a knockout victory twenty-nine seconds into the second round.[11] Prior to this match, Matthysse had never been knocked down by any opponent, although he had suffered a tenth-round technical knockout in May 2006 at the hands of Paul Williams. (The Cintrón-Matthysse bout was the main undercard of a WBO title bout between challenger Williams and incumbent Margarito)

Cintrón and Williams were scheduled for an IBF/WBO title unification bout on February 2, 2008. But as a result of an injury to Cintrón's right hand sustained during his bout with Jesse Feliciano the fight has been canceled.[12]

Cintrón lost his IBF title to Antonio Margarito by knockout on an April 12 broadcast of HBO's World Championship Boxing in a rematch from Atlantic City, NJ, on the undercard of Miguel Cotto vs. Alfonso Gomez. In the sixth round, Cintrón fell to the canvas after receiving a one-two punch to the head and a body shot and did not get up before the referee concluded the protective count.[13] It was the first time he had lost a professional fight by a knockout (his previous loss to Margarito being scored as a technical knockout).

Cintrón returned to action against Lovemore N'dou, competing in an eliminatory fight sanctioned by the IBF. The fight took place on November 15, 2008, in a card held in Nashville, Tennessee. Early in the fight, Cintrón began on the offensive, while N'dou clinched regularly.[14] This pattern slowed the fight's tempo throughout the contest. In the third round, Cintrón connected a solid punch, N'dou tried to counterattack and exchange but was ineffective, eventually returning to a strategy based on holds.[14] The numerous clinches resulted in head butts, the referee issued numerous warnings to N'dou for these, eventually deducting a point in the ninth. In the tenth round, an accidental headbutt opened a laceration over Cintrón's eyebrow. In the eleventh round, he was able to injure N'dou with a punch, but did not continue pressing the offensive. Subsequently, the judges awarded Cintrón scores of 117-110, 115-112 and 116-111.[14]

Cintrón's victory over N'dou made him Joshua Clottey's mandatory challenger. The fight was discussed and preliminary agreements scheduled it for February 21, 2009.[15] However, after receiving a more lucrative offer, Cintrón decided to fight Sergio Gabriel Martínez for the World Boxing Council's interim light middleweight championship.[15] The bout took place on February 14, 2009, and ended in a controversial majority draw. The opening rounds held no meaningful action, with Cintrón holding an early edge due to his aggression and cleaner punching. By the fourth round, Martínez started moving around with his hands down in an effort to draw Cintrón into a mistake, but to no avail, as there continued to be more clinching than punching. Martínez opened a cut over Cintrón's left eye early in the fifth round. Late in the seventh round, a left hand to the head hurt Cintrón, and after backing into the ropes, he went down to his knees before Martínez could attack. Cintrón claimed he was headbutted while the referee Frank Santore continued the count. After Cintrón's protests and a lot of confusion in the ring, Santore allowed the bout to continue, saying Cintrón was up at nine and that he never stopped the fight. Martínez went after Cintrón once the eighth round commenced and taunted him after landing punches to the head. Cintrón responded with sustained action of his own, but it was Martínez ending the round with another left hand to the head. The ninth round was favorable for Martínez, but Cintrón rebounded in the tenth round. Martínez lost a point for a punch to the back of the head in the final round. Scores were 116-110 for Martínez and 113-113 draw.[16]

On May 30, 2009 Cintron defeated Alfredo Angulo (15-0) by a unanimous decision.

Wins over highly regarded Alfredo Angulo and lesser-known Juliano Ramos put Cintrón into position to fight Paul "The Punisher" Williams, thought to be one of the top pound-for-pound fighters in the sport. This fight took place on May 8, 2010. Although Williams often averages over one hundred punches per round, Cintrón was able to neutralize his punch output over the first three rounds by effectively jabbing and countering with his right hand. This resulted in a very technical three rounds of boxing and, not surprisingly, voluminous jeers from the spectating crowd. In the fourth round, however, the combatants began to exchange punches, each landing hard power punches on one another. One such exchange caused an entanglement between the fighters, sending Williams to the canvas and Cintrón through the ropes, where he landed on a media table abutting the exterior of the ring. He then fell to the floor and was immediately attended to by ringside doctors. Boxing rules dictate that a fighter, in these circumstances, is afforded a five-minute period with which to recover; however, given that Cintrón was advised by doctors not to move, the fight was called and Cintrón was removed from the arena bound to a stretcher. Since three rounds had been completed, the fight became official and Williams was declared the winner by split decision.

On July 9, 2010, Cintrón lost a unanimous decision to Carlos Molina, who had a 18-4-2 record going into the fight.

On August 12, 2011, Cintrón won a unanimous decision over Antwone Smith.

On November 26, 2011, Cintrón was knocked out in five rounds by Saul Alvarez.